Open-chain nitrogen compounds. Part VIII. 1-(2′-Acetylphenyl)-3-alkyltriazenes with reactive substituents in the alkyl group: synthons for five- and six-membered nitrogen heterocycles

Abstract

The synthesis of a series of 1-(2′-acetylphenyl)-3-alkyltriazenes, with electron-withdrawing α-substituents in the alkyl moiety, is described. The structure of the products from the reaction of 2-acetylbenzenediazonium ion (1) with methylamine has been reinvestigated using high field (360 MHz) nmr spectroscopy. The 2-acetylphenyltriazenes undergo cyclization to five-or six-membered heterocycles, depending on the substituent present in the alkyl group. Attack by N-3 of the triazene on the ortho-acetyl group leads to a 4-methylenetriazine, which can undergo oxidation to a benzotriazinone or acid-catalysed rearrangement to a 4-alkylaminocinnoline. The reaction of 1 with alkylamines also affords red products, which have been identified as 4-arylazomethylenetriazines and fully characterized by 1H and 13C nmr spectroscopy. The formation of these azo compounds is consistent with the enamine character of the 4-methylenetriazines. The introduction of a CO2Et group at the α-position of the alkyl moiety provides a competing cyclization pathway to give a five-membered heterocycle, a 5-hydroxytriazole. In this case the cyclization pathway to the six-membered heterocycle is favoured, whereas the introduction of a CN group can lead to exclusive triazole formation. The reaction of the nitrile (2c) with alumina in chloroform leads to a new heterocycle, a quinazolinotriazole; in DMSO solution, 2c prefers the alternative pathway and gives only the 3-cyanomethyl-4-methylenetriazine. Introduction of a benzoyl group in the alkyl moiety gives an unstable triazene; it reacts with alumina in chloroform to give a triazole, which in turn rearranges to a novel diazoalkane.